The US senator for Vermont promised supporters gathered in California on Tuesday night that he would continue campaigning for next week's primary in Washington DC, and on to the Democratic Party's national convention in July.

Clinton declared victory after picking up primary wins in New Jersey, South Dakota, and New Mexico, just a day after an Associated Press count - including unpledged superdelegates - called the race in her favour. She also won in California, the largest state in the United States.

"We are going to fight hard to win the primary in Washington DC and then we take our fight for social, economic, racial and environmental justice to Philadelphia," Sanders said, referring to the city where Democrats will gather for their party convention on July 25.

"The fight in front of us is a very, very steep fight, but we will continue to fight for every vote and every delegate."

Sanders won primaries in Montana and North Dakota on Tuesday night [Lucy Nicholson/Reuters]

'Political revolution'

Sanders has gone from a candidate once considered an outsider by sections of the US media to one who has gone toe-to-toe with Clinton.

"All of you know that when we began this campaign just over a year ago, we were considered to be a fringe campaign. But over the last year, I think that has changed just a little bit," he said.

The current AP tally of pledged delegates puts Clinton ahead with 1,926 to 1,615 for Sanders.

When superdelegates are taken into account, Clinton has 2,497 delegates to 1,663 for Sanders - enough to secure her the nomination.

Sanders has built his campaign on a promise of tackling economic inequality, which he says is caused by large corporations and greedy billionaires, as well as a political class that has done nothing to redress it.

"We will break up the major banks on Wall Street and we will join the rest of the industrialised world and guarantee healthcare," he said, thanking his supporters for being part of a "political revolution".

"We will tell the billionaire class and Corporate America that they will start paying their fair share."

Sanders also promised that he would not allow the presumptive Republican nominee, Donald Trump, to become president, condemning his "bigotry" against Mexicans, Muslims, and African-Americans.

"From the very beginning Bernie Sanders has said he's in this to run a political revolution, and he wants to get his message out at the convention. He wants the superdelegates to think about who can beat Donald Trump," political science professor Peter Mathews told Al Jazeera.

US President Barack Obama congratulated Clinton on securing the nomination, and is set to meet Sanders at the White House on Thursday.

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